EXISTENCE OF A HEAD-KIDNEY. 619 



a groove in the peritoneal epithelium, which is continued back- 

 ward as a primitively solid rod in the space between the Wolf- 

 fian duct and peritoneal epithelium. 



In our preliminary account we stated 1 , in accordance with 

 the general view, that the Miillerian duct was formed as a groove, 

 or elongated involution of the peritoneal epithelium adjoining 

 the Wolffian duct. We have now reason to believe that this is 

 not the case. In the earliest condition of the Mullerian duct 

 which we have been able to observe, it consists of three succes- 

 sive open involutions of the peritoneal epithelium, connected 

 together by more or less well-defined ridge-like thickenings of 

 the epithelium. We believe, on grounds hereafter to be stated, 

 that the whole of this formation is equivalent to the head-kidney 

 of the Ichthyopsida. The head-kidney, as we shall continue to 

 call it, takes its origin from the layer of thickened epithelium 

 situated near the dorsal angle of the body-cavity, close to the 

 Wolffian duct, which has been known since the publication of 

 Waldeyer's important researches as the germinal epithelium. 

 The anterior of the three open involutions or grooves is situated 

 some little distance behind the front end of the Wolffian duct. 

 It is simply a shallow groove in the thickest part of the germinal 

 epithelium, and forms a corresponding projection into the ad- 

 jacent stroma. In front the projection is separated by a con- 

 siderable interval from the Wolffian duct ; but near its hinder- 

 most part it almost comes into contact with the Wolffian duct. 

 The groove extends in all for about five of our sections, and 

 then terminates by its walls becoming gradually continued into 

 a slight ridge-like thickening of the germinal epithelium. The 

 groove arises as a simple depression in a linear area of thick- 

 ened germinal epithelium. The linear area is, however, con- 

 tinued very considerably further forward than the groove, and 

 sometimes exhibits a slight central depression, which might be 

 regarded as a forward continuation of the groove. The passage 

 from the groove to the ridge may best be conceived by sup- 

 posing the groove to be suddenly filled up, so as to form a solid 

 ridge pointing inwards towards the Wolffian duct. 



The ridge succeeding the first groove is continued for about 

 six sections, and is considerably more prominent at its posterior 



1 Proceedings of Royal Society, 1878. 



4O 2 



